1. Field of the Invention
The present invention relates to a damper unit and more particularly to a damper unit which is preferable for disposition between springs of a suspension system of a motor vehicle not only to damp the vibration of the springs but also to control the height of a vehicle so as to stay at a desired height.
2. Description of the Related Art
A technique described in Patent Document is known as a damper unit for a motor vehicle. In the technique described in JP-A-2007-292284, a working or hydraulic oil (a hydraulic fluid) is supplied to a lower chamber of a cylinder in association with a vertical movement of a piston so as to push a piston rod upwards to cause it to extend to raise the vehicle height on one hand. On the other hand, when the vehicle height reaches a predetermined vehicle height, by a leveling port opened in a pump rod being caused to communicate with the lower chamber, the vehicle height is designed to be prevented from being raised any further.
In this way, in the technique described in JP-A-2007-292284, the vehicle height to be held is determined by the position of the leveling port so opened for releasing the pumping operation. Therefore, JP-A-10-211810 proposes a technique for controlling an axial height in which a female screw member and a male screw member made to mesh with the female screw member are accommodated in an interior of a piston rod for connection with a pump rod (a pump tube), and a pumping operation is designed to be released by moving the female screw member in an axial direction by an actuator.
In this way, in the technique described in JP-A-10-211810, since the axial height control is implemented by adopting the configuration in which the pumping operation is released by causing the female screw member to move in the axial direction or, specifically, causing the female screw member to rotate by the actuator to cause the male screw member meshing with the female screw member to move or, more specifically, converting the rotational motion of the female screw member by the actuator into the linear motion of the male screw member, not only does the configuration become complex, but also a space is required within the piston rod which corresponds to a distance over which not only the female screw member but also the male screw member moves, this making it difficult for the damper unit to be laid out within a limited space such as one within a wheel house when trying to dispose the damper unit in such a space.